Ceramic heating elements

ABSTRACT

New ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can provide a reduced cross-sectional dimension across element regions that interface with electrical lead(s) as well as a more secure engagement of an electrical lead. Heating elements can be highly useful in a variety of application, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs.

The present application claims the benefit of U.S. provisionalapplication No. 61/009,381 filed Dec. 29, 2007, which is incorporated byreference herein in its entirety.

BACKGROUND

1. Field of the Invention

In one aspect, ceramic heating elements are provided that have arecessed portion for receiving an electrical lead. Such ceramic heatingelements can more secure engagement of the heating element with anelectrical lead. In a further aspect, ceramic heating elements areprovided that have a conductive zone of substantially equal orincreasing cross-section along a length of the element. The presentheating elements are useful in a variety of application, including e.g.for fuel ignition for gas cooking appliances as well as vehicular glowplugs that have strict space constraints.

2. Background

Ceramic materials have enjoyed great success as heating elements(includes igniters) in e.g. gas-fired furnaces, stoves and clothesdryers. Ceramic heating element production includes constructing anelectrical circuit through a ceramic component a portion of which ishighly resistive and rises in temperature when electrified by a wirelead. See, for instance, U.S. Patent Publication 2006/0131295 and U.S.Pat. Nos. 6,028,292; 5,801,361; 5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with ahighly resistive “hot zone” at the heating element tip with one or moreconductive “cold zones” providing to the hot zone from the opposingheating element end. One currently available igniter, the Mini-Heatingelement, available from Norton Igniter Products of Milford, N.H., isdesigned for 12 volt through 120 volt applications and has a compositioncomprising aluminum nitride (“AlN”), molybdenum disilicide (“MoSi₂”),and silicon carbide (“SiC”).

Since these heating elements are resistively heated, each of its endsmust be electrically connected to a conductive lead, typically a copperwire lead. Ceramic heating elements have been connected to electricalcontact by direct welding or brazing to wire or by brazing to anintermediate metal lead frame which is then welded or brazed to wire.See U.S. Pat. Nos. 7,241,975 and 6,933,471.

For heating elements that have cylindrical or other non-rectangularcross-section configurations, such attachment of electrical contacts canresult in an increase in the diameter of the insulating section (wherethe electrical leads interface with the heating element). Such increaseddimensions can be problematic for a number of applications, such asappliances or automotive environments where tight specifications mayexists for the outer dimensions of the heating element block of theheating element. Additionally, separation of the electrical lead fromthe heating element can result in device failure.

It thus would be desirable to have new heating element systems. It wouldbe particularly desirable to have new heating elements that havecylindrical or other non-rectangular cross-sectional configurations andthat have comparatively narrow cross-sectional dimensions across regionsthat interface with electrical contacts. It would be further desirableto have new heating elements that have secure engagement of anelectrical lead to the heating element.

SUMMARY

In one aspect, ceramic heating elements are provided that have arecessed portion for receiving an electrical lead. Such ceramic heatingelements can provide a reduced cross-sectional dimension across elementregions that interface with electrical lead(s) as well as a more secureengagement of the lead(s) to the heating device. Consequently, heatingelements can be highly useful in a variety of applications, includinge.g. for fuel ignition for gas cooking appliances as well as vehicularglow plugs.

In a preferred aspect, a heating element may comprise at least onerecess (e.g. hole) that can receive an electrical lead, where the recessis positioned at a bottom face of the heating element, although therecess also suitably may be situated in other regions of a heatingelement, such as a side portion of an element.

In certain aspects, the recess may be tapered, e.g. inwardly tapered(decreasing cross-sectional area), which can further secure anengagement of an electrical lead with the heating element.

Preferably, a conductive zone (i.e. region of relatively lowresistivity) of the heating element forms at least a portion of the wallsurface of the recess. As a consequence, power from an electrical leadnested within the recess can flow through the heating element via suchconductive zone.

In a further aspect, ceramic heating elements are provided that have aconductive zone of substantially equal or increasing cross-section froma proximal end of the heating element along the element length. Inparticular, the cross-sectional dimension of the conductive zone thatforms at least a portion of the wall surface of the recess for receivingan electrical lead will have a cross-sectional dimension at a portionthat contacts the recess that is substantially equal to or greater thanthe cross-section dimension of that same conductive zone further alongthat conductive zone's length.

It has been found that such conductive zone configurations can avoidundesired warpage upon sintering of the heating element.

Preferred heating elements of the invention have an outer orsubstantially U-shaped or L-shaped electrical path, i.e. where theelectrical path extends from (i) an outer conductive zone to (ii) an hotor ignition zone and then through (iii) a second outer conductive zone.Such an outer or U-shaped or L-shaped electrical path is different thanand distinguished from a co-axial path that contains an interior firstconductive zone that is encased by an outer conductive zone.

Particularly preferred heating elements of the invention may havecylindrical or other non-rectangular cross-section configurations. In apreferred aspect, preferred heating elements of the invention have arounded cross-sectional shape along at least a portion of the heatingelement length (e.g., the length extending from where an electrical leadis affixed to the heating element to a resistive hot zone). Moreparticularly, preferred heating elements may have a substantially oval,circular or other rounded cross-sectional shape for at least a portionof the heating element length, e.g. at least about 10 percent, 40percent, 60 percent, 80 percent or 90 percent of the heating elementlength, or the entire heating element length. A substantially circularcross-sectional shape that provides a rod-shaped heating element isparticularly preferred. The invention also provides heating elementsthat have non-rounded or non-circular cross-sectional shapes for atleast a portion of the heating element length.

Preferred heating elements comprise multiple regions of differingelectrical resistivity, i.e. preferred ceramic heating elements maycomprise a first conductive zone, a resistive hot zone, and a secondconductive zone, all in electrical sequence. Heating elements of theinvention may have a variety of electrical configurations. As discussed,in preferred systems, the heating element may have a substantiallyU-shaped electrical path, e.g. where opposing conductive zones areseparated by an interposed hot or ignition zone.

Ceramic heating elements of the invention can be employed at a widevariety of nominal voltages, including nominal voltages of 6, 8, 10, 12,24, 120, 220, 230 and 240 volts.

As mentioned, the heating elements of the invention are useful forignition in a variety of devices and heating systems. More particularly,heating systems are provided that comprise a sintered ceramic heatingelement as described herein. Specific heating systems include appliancessuch as gas cooking units, heating units for commercial and residentialbuildings. Vehicular (e.g. automotive, watercraft) glow plugs alsoprovided that comprise a sintered ceramic heating element as describedherein.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 show schematically preferred heating element systems;

FIG. 5 shows in a cut-away view a further preferred heating element; and

FIG. 6 (which includes FIGS. 6A-6C) shows plan views of a furtherpreferred heating element. FIG. 6B is a view sliced along line B-B ofFIG. 6A, and FIG. 6C is a view sliced along line C-C of FIG. 6A.

DETAILED DESCRIPTION

As discussed above, in one aspect, ceramic heating element systems areprovided that include new configurations for mating of electrical leadcomponents. In a further aspect, ceramic heating element systems areprovided that include conductive region(s) that can provide notablebenefits, including reduced warpage upon sintering. Preferred ceramicheating elements of the invention having a substantially outer orU-shaped or L-shaped electrical path.

Referring now to the drawings, FIGS. 1 through 4 show in a schematiccut-away view a preferred heating element 10 where conductive zones 12Aand 12B mate with interposed hot (ignition) zone 14 to thereby form anelectrical pathway. As can be seen, outer conductive zones 12A and 12Btogether with interposed hot (ignition) zone form a substantiallyU-shaped or L-shaped electrical pathway that traverses an outer orperimeter portion of the heating element 10.

Conductive zone 12A defines in part recess 16 that engages withelectrical lead 18 during use of element 10. In preferred systems,heating element 10 may be encased with a metal fixture 20 and affixedtherethrough, e.g. via a metal braze 22. The interior region 24 encasedby conductive zones 12A, 12B and ignition zone 14 may be void or mayhave an insulative (heat sink) composition.

It also may be preferred to include an exterior insulative layer 25 onheating element portions that contact metal fixture 20. Such an exteriorinsulative layer may be suitably formed by dip coating or otherapplication of an insulative ceramic composition.

FIG. 5 shows in a partial cut-away view a particularly preferred heatingelement where conductive zone 12A forms a portion of walls 16A of recess16 that receives an electrical lead. In this preferred configuration,conductive zone 12A and 12B mate with interposed hot (ignition) zone 14to thereby form an electrical pathway. The heating element also includescentral insulator region 24 with outer insulator 25A that encases atleast a portion of the first conductive zone 12A as well as insulator25B that encases at least a portion of conductive zone 12B.

As depicted in FIG. 5, in a preferred configuration, recess 16 contactsconductive zone 12A whereby walls 16A of the recess are formed byconductive zone 12A. In other less preferred embodiments, the entiresurface of the walls that define recess 16 are part of the conductivezone 12A.

In the depicted preferred configuration, by only a portion of the wallsthat define recess 16 being part of the conductive zone 12A, thatconductive zone 12A can have a substantially equal or increasingcross-section along a length of the element. Thus, as shown in FIG. 5,the cross-section dimension of conductive zone 12A at the elementproximal end (as shown by dimension a) is substantially the same as orless than the cross-sectional dimension of that conductive zone for thesubstantial portion of that zone's length (length y as depicted in FIG.5), e.g. for at least about 50, 60, 70, 80, 90, 95 or even 100 percentof that zone's length y as shown in FIG. 5. Thus, as shown in FIG. 5,dimension a will be the about the same as or less than the depicteddimensions a′ or a″. As referred to herein, references to the firstconductive zone having “substantially the same” or “about the same” (orother similar phrase) cross-section along its length means that thecross-section dimension (such as a relative to a′ and a″ as shown inFIG. 5) does not vary by more than 5, 10 or 20 percent. In certainaspects, references to the cross-section dimension of the conductivezone does not include the interface of that conductive zone and themating ignition zone.

As discussed above, this configuration of the first conductive zonecross-sectional dimension has provided notable benefits, includingreduced undesired warpage upon sintering of the heating element.

FIG. 5 also shows a preferred configuration of recess 16, where recess16 inwardly tapers, i.e. recess 16 has a decreased cross-section alongits length. Such a tapered configuration can provide more secureengagement of an electrical lead nested within the recess.

In use, an electrical lead is nested within recess 16 and provides powerthrough the depicted electrical pathway (see pathway as shown by arrowsin FIG. 5) that extends from conductive zone 12A to hot (ignition) zone14. Interior ceramic insulator 24 can provide further mechanicalstrength to the heating element.

FIGS. 6A through 6C show a further heating element 10 in a preferredconfiguration where only a portion of recess 16 contacts conductive zone12A. That is, only a portion of the surface that defines recess 16 is acomponent of conductive zone 12A. In the system depicted in FIG. 6, thebalance of the walls defining recess 16 is a component of insulatorregion 24. In exemplary preferred systems, up to about 20, 30, 40, 50,60, 70, 80 or 90 percent of the surface area of the walls of recess 16may be a component of the component of conductive zone 12A, with thebalance of the surface area of the walls of recess 16A being a componentof the central or interior insulator region 24.

As with the element of FIG. 5, in use, an electrical lead is nestedwithin recess 16 and provides power through the depicted electricalpathway from conductive zone 12A, through hot (ignition) zone 14 andthen through conductive zone 12B to provide a substantially U-shaped orL-shaped outer electrical pathway.

As shown in FIGS. 5 and 6A, in preferred systems, the length of firstconductive zone 12A that contacts recess 16 is greater than the lengthof second conductive zone 12B that is on the distal side of resistive(ignition) zone 14. For example, in certain preferred configurations,the length of second conductive zone 12B (shown as y′ in FIG. 5) is nomore than 90, 80, 70, 60, 50, 40, 30, 20 or even 10 percent the lengthof the first conductive zone 12A (length y is FIG. 5). In certainpreferred configurations, second conductive zone 12B does not contain orcontact recess 16, as shown in FIGS. 5, 6A and 6B.

As discussed above, and exemplified in FIGS. 5 and 6, preferably, atleast a substantial portion of the heating element length has a roundedcross-sectional shape along at least a portion of the heating elementlength, such as length a shown in FIG. 6A. FIGS. 5 and 6 depict aparticularly preferred configuration where heating element 10 has asubstantially circular cross-sectional shape for about the entire lengthof the heating element to provide a rod-shaped heating element. However,as discussed above, preferred systems also include those where only aportion of the heating element has a rounded cross-sectional shape, suchas where up to about 10, 20, 30, 40, 50, 60, 70 80 or 90 percent of theheating element length, e.g. where in such designs, the balance of theheating element length may have a profile with exterior edges.

Also, while a rounded cross-sectional shape is preferred for manyapplications, preferred heating elements of the invention also may havea non-rounded or non-circular cross-sectional shape for at least aportion of the heating element length, e.g. where up to or at leastabout 10, 20, 30, 40, 50, 60, 70 80 or 90 percent of the heating elementlength (as exemplified by heating element length a in FIG. 6A) has across-sectional shape that is non-rounded or non-circular, or where theentire heating element length (as a heating element length isexemplified by length a in FIG. 6A) has a cross-sectional shape that isnon-rounded or non-circular.

Dimensions of heating elements of the invention may vary widely and maybe selected based on intended use of the heating element. For instance,the length of a preferred heating element (length a in FIG. 6A) suitablymay be from about 0.5 to about 5 cm, more preferably from about 1 about3 cm, and the heating element maximum cross-sectional width may suitablybe from about (width b in FIG. 6A) suitably may be from about 0.2 toabout 3 cm.

Similarly, the lengths of the conductive and hot zone regions also maysuitably vary. Preferably, the length of first conductive zone (length cin FIG. 6A and y in FIG. 5) of a heating element depicted in FIG. 6A maybe from 0.2 cm to 2, 3, 4, or 5 more cm. More typical lengths of thefirst conductive zone will be from about 0.5 to about 5 cm. The heightof a hot zone (length d in FIG. 6A) may be from about 0.1 to about 2, 3,4 or 5 cm, with a total hot zone electrical path length (shown as thedashed line in FIG. 6A) of about 0.5 to 5 or more cm, with a total hotzone path length of up to about 0.5 to 1, 2 or 3 cm generally preferred.

In preferred systems, the hot or resistive zone of a heating element ofthe invention will heat to a maximum temperature of less than about1450° C. at nominal voltage; and a maximum temperature of less thanabout 1550° C. at high-end line voltages that are about 110 percent ofnominal voltage; and a maximum temperature of less than about 1350° C.at low-end line voltages that are about 85 percent of nominal voltage.

A variety of compositions may be employed to form a heating element ofthe invention. Generally preferred hot zone compositions comprise atleast three components of 1) conductive material; 2) semiconductivematerial; and 3) insulating material. Conductive (cold) and insulative(heat sink) regions may be comprised of the same components, but withthe components present in differing proportions. Typical conductivematerials include e.g. molybdenum disilicide, tungsten disilicide,nitrides such as titanium nitride, and carbides such as titaniumcarbide. Typical semiconductors include carbides such as silicon carbide(doped and undoped) and boron carbide. Typical insulating materialsinclude metal oxides such as alumina or a nitride such as AlN and/orSi₃N₄.

As referred to herein, the term electrically insulating materialindicates a material having a room temperature resistivity of at leastabout 10¹⁰ ohms-cm. The electrically insulating material component ofheating elements of the invention may be comprised solely or primarilyof one or more metal nitrides and/or metal oxides, or alternatively, theinsulating component may contain materials in addition to the metaloxide(s) or metal nitride(s). For instance, the insulating materialcomponent may additionally contain a nitride such as aluminum nitride(AlN), silicon nitride, or boron nitride; a rare earth oxide (e.g.yttria); or a rare earth oxynitride. A preferred added material of theinsulating component is aluminum nitride (AlN).

As referred to herein, a semiconductor ceramic (or “semiconductor”) is aceramic having a room temperature resistivity of between about 10 and10⁸ ohm-cm. If the semiconductive component is present as more thanabout 45 v/o of a hot zone composition (when the conductive ceramic isin the range of about 6-10 v/o), the resultant composition becomes tooconductive for high voltage applications (due to lack of insulator).Conversely, if the semiconductor material is present as less than about10 v/o (when the conductive ceramic is in the range of about 6-10 v/o),the resultant composition becomes too resistive (due to too muchinsulator). Again, at higher levels of conductor, more resistive mixesof the insulator and semiconductor fractions are needed to achieve thedesired voltage. Typically, the semiconductor is a carbide from thegroup consisting of silicon carbide (doped and undoped), and boroncarbide. Silicon carbide is generally preferred.

As referred to herein, a conductive material is one which has a roomtemperature resistivity of less than about 10⁻² ohm-cm. If theconductive component is present in an amount of more than 35 v/o of thehot zone composition, the resultant ceramic of the hot zone composition,the resultant ceramic can become too conductive. Typically, theconductor is selected from the group consisting of molybdenumdisilicide, tungsten disilicide, and nitrides such as titanium nitride,and carbides such as titanium carbide. Molybdenum disilicide isgenerally preferred.

In general, preferred hot (resistive) zone compositions include (a)between about 50 and about 80 v/o of an electrically insulating materialhaving a resistivity of at least about 10¹⁰ ohm-cm; (b) between about 5and about 45 v/o of a semiconductive material having a resistivity ofbetween about 10 and about 10⁸ ohm-cm; and (c) between about 5 and about35 v/o of a metallic conductor having a resistivity of less than about10⁻² ohm-cm. Preferably, the hot zone comprises 50-70 v/o electricallyinsulating ceramic, 10-45 v/o of the semiconductive ceramic, and 6-16v/o of the conductive material. A specifically preferred hot zonecomposition for use in heating elements of the invention contains 10 v/oMoSi₂, 20 v/o SiC and balance AlN or Al₂O₃.

As discussed, heating elements of the invention contain a relatively lowresistivity cold zone region in electrical connection with the hot(resistive) zone and which allows for attachment of wire leads to theheating element. Preferred cold zone regions include those that arecomprised of e.g. AlN and/or Al₂O₃ or other insulating material; SiC orother semiconductor material; and MoSi₂ or other conductive material.However, cold zone regions will have a significantly higher percentageof the conductive and semiconductive materials (e.g., SiC and MoSi₂)than the hot zone. A preferred cold zone composition comprises about 15to 65 v/o aluminum oxide, aluminum nitride or other insulator material;and about 20 to 70 v/o MoSi₂ and SiC or other conductive andsemiconductive material in a volume ratio of from about 1:1 to about1:3. For many applications, more preferably, the cold zone comprisesabout 15 to 50 v/o AlN and/or Al₂O₃, 15 to 30 v/o SiC and 30 to 70 v/oMoSi₂. For ease of manufacture, preferably the cold zone composition isformed of the same materials as the hot zone composition, with therelative amounts of semiconductive and conductive materials beinggreater.

A specifically preferred cold zone composition for use in heatingelements of the invention contains 20 to 35 v/o MoSi₂, 45 to 60 v/o SiCand balance either AlN and/or Al₂O₃.

For any of the ceramic compositions (e.g. insulator, conductivematerial, semiconductor material, resistive material), the ceramiccompositions may comprise one or more different ceramic materials (e.g.SiC, metal oxides such as Al₂O₃, nitrides such as AlN, Mo₂Si₂ and otherMo-containing materials, SiAlON, Ba-containing material, and the like).Alternatively, distinct ceramic compositions (i.e. distinct compositionsthat serve as insulator, conductor and resistive (ignition) zones in asingle heating element) may comprise the same blend of ceramic materials(e.g. a binary, ternary or higher order blend of distinct ceramicmaterials), but where the relative amounts of those blend membersdiffer, e.g. where one or more blend members differ by at least 5, 10,20, 25 or 30 volume percent between the respective distinct ceramiccompositions.

A heat sink or insulator may suitably mate with a conductive zone or ahot zone, or both. Preferably, a sintered insulator region has aresistivity of at least about 10¹⁴ ohm-cm at room temperature and aresistivity of at least 10⁴ ohm-cm at operational temperatures and has astrength of at least 150 MPa. Preferably, an insulator region has aresistivity at operational (ignition) temperatures that is at least 2orders of magnitude greater than the resistivity of the hot zone region.Suitable insulator compositions comprise at least about 90 v/o of one ormore aluminum nitride, alumina and boron nitride. A specificallypreferred insulator composition of an heating element of the inventionconsists of 60 v/o AlN; 10 v/o Al₂O₃; and balance SiC. Another preferredheat sink (insulator) composition for use with an heating element of theinvention contains 80 v/o AlN and 20 v/o sic.

For certain systems, it may be desirable to include a power booster orenhancement zone of intermediate resistance in the electrical circuitpathway between the most conductive portions of that pathway and thehighly resistive (hot) regions of that pathway. Such booster zones ofintermediate resistance are described in U.S. Patent applicationPublication 2002/0150851 to Willkens. Generally, booster zones will havea positive temperature coefficient of resistance (PTCR) and anintermediate resistance that will permit i) effective current flow to ahot zone, and ii) some resistance heating of the booster region duringuse of the igniter, although preferably the booster zone will not heatto as high temperatures as the hot zone during use of the heatingelement.

If employed in a heating element, preferred booster zone compositionsmay comprise the same materials as the conductive and hot zone regioncompositions, e.g. preferred booster zone compositions may comprise e.g.AlN and/or Al₂O₃, or other insulating material; SiC or othersemiconductor material; and MoSi₂ or other conductive material. Abooster zone composition typically will have a relative percentage ofthe conductive and semiconductive materials (e.g., SiC and MoSi₂) thatis intermediate between the percentage of those materials in the hot andcold zone compositions. A preferred booster zone composition comprisesabout 60 to 70 v/o aluminum nitride, aluminum oxide, or other insulatormaterial; and about 10 to 20 v/o MoSi₂ or other conductive material, andbalance a semiconductive material such as SiC. A specifically preferredbooster zone composition for use in igniters of the invention contains14 v/o MoSi₂, 20 v/o SiC and balance v/o Al₂O₃. A specifically preferredbooster zone composition for use in igniters of the invention contains17 v/o MoSi₂, 20 v/o SiC and balance Al₂O₃. A further specificallypreferred booster zone composition for use in igniters of the inventioncontains 14 v/o MoSi₂, 20 v/o SiC and balance v/o AlN. A still fartherspecifically preferred booster zone composition for use in igniters ofthe invention contains 17 v/o MoSi₂, 20 v/o SiC and balance AlN.

The processing of the ceramic component (i.e. green body and sinteringconditions) and the preparation of the heating element from thedensified ceramic can be done by conventional methods and as discussedabove. See U.S. Pat. No. 5,786,565 to Wilkens and U.S. Pat. No.5,191,508 to Axelson et al.

A preferred fabrication method includes use of injection moldingtechniques. Thus, for instance, a base element may be formed byinjection introduction of a ceramic material having a first resistivity(e.g. ceramic material that can function as an insulator or heat sinkregion) into a mold element that defines a desired base shape such as arod shape. The base element may be removed from such first mold andpositioned in a second, distinct mold element and ceramic materialhaving differing resistivity—e.g. a conductive ceramic material—can beinjected into the second mold to provide conductive region(s) of theigniter element. In similar fashion, the base element may be removedfrom such second mold and positioned in a yet third, distinct moldelement and ceramic material having differing resistivity—e.g. aresistive hot zone ceramic material—can be injected into the third moldto provide resistive hot or ignition region(s) of the igniter element.

Alternatively, rather than such use of a plurality of distinct moldelements, ceramic materials of differing resistivities may besequentially advanced or injected into the same mold element. Forinstance, a predetermined volume of a first ceramic material (e.g.ceramic material that can function as an insulator or heat sink region)may be introduced into a mold element that defines a desired base shapeand thereafter a second ceramic material of differing resistivity may beapplied to the formed base.

Ceramic material may be advanced (injected) into a mold element as afluid formulation that comprises one or more ceramic materials such asone or more ceramic powders.

For instance, a slurry or paste-like composition of ceramic powders maybe prepared, such as a paste provided by admixing one or more ceramicpowders with an aqueous solution or an aqueous solution that containsone or more miscible organic solvents such as alcohols and the like. Apreferred ceramic slurry composition for extrusion may be prepared byadmixing one or more ceramic powders such as MoSi₂, Al₂O₃, and/or AlN ina fluid composition of water optionally together with one or moreorganic solvents such as one or more aqueous-miscible organic solventssuch as a cellulose ether solvent, an alcohol, and the like. The ceramicslurry also may contain other materials e.g. one or more organicplasticizer compounds optionally together with one or more polymericbinders.

A wide variety of shape-forming or inducing elements may be employed toform an igniter element, with the element of a configurationcorresponding to desired shape of the formed igniter. For instance, toform a rod-shaped element, a ceramic powder paste may be injected into acylindrical die element. To form a stilt-like or rectangular-shapedigniter element, a rectangular die may be employed.

After advancing ceramic material(s) into a mold element, the definedceramic part suitably may be dried e.g. in excess of 50° C. or 60° C.for a time sufficient to remove any solvent (aqueous and/or organic)carrier.

Thereafter, the heating element may be further densified (e.g. togreater than 95, 96, 97, 98 or 99 percent) by thermal treatment such asin excess of 1500° C., 1600° C., 1700° C. or 1800° C. A single ormultiple thermal treatments may be conducted as desired to achieve finaldensities.

Heating elements of the invention may be used in many applications,including gas phase fuel ignition applications such as furnaces andcooking appliances, baseboard heaters, boilers, and stove tops. Inparticular, an heating element of the invention may be used as anignition source for stove top gas burners as well as gas furnaces.

As discussed above, heating elements of the invention will beparticularly useful where rapid ignition is beneficial or required, suchas in ignition of a heating fuel (gas) for an instantaneous water heaterand the like. Heating elements also may be employed as glow plug in avariety of vehicles (automotive, watercraft).

The following non-limiting examples are illustrative of the invention.All documents mentioned herein are incorporated herein by reference intheir entirety.

EXAMPLE 1 Heating Element Fabrication

Powders of a resistive composition (20 vol % MoSi₂, 5 vol % SiC, 74 vol% Al₂O₃ and 1 vol % Gd₂O₃), a conductive composition (28 vol % MoSi₂, 7vol % SiC , 64 vol % Al₂O₃ and 1 vol % Gd₂O₃) and an insulatingcomposition (10 vol % MoSi₂, 89 vol % Al₂O₃ and 1 vol % Gd₂O₃) are mixedwith 10-16 wt % organic binder (about 6-8 wt % vegetable shortening, 2-4wt % polystyrene and 2-4 wt % polyethylene) to form three pastes withabout 62-64 vol % solids loading. The three pastes are loaded into thebarrels of a co-injection molder. A first shot filled a cavity that hasan hour-glass shaped cross-section with the insulating paste forming thesupporting base. The part is removed from the first cavity and placed ina second cavity. A second shot fills the bottom half of the volumebounded by the first shot and the cavity wall with the conductive paste.The part is removed from the second cavity and placed in a third cavity.A third shot filled the volume bounded by the first shot, second shotand the cavity wall with resistive paste forming a hair-pin shapedresistor separated by the insulator and connected to conductive legs andhaving the configuration shown in FIG. 5. The part is then thermallydebindered in Ar or N₂ at 500° C. for 24 h to remove the remainingbinder and densified to 95-97% of theoretical at 1750° C. in Argon at 1atm pressure.

The invention has been described in detail with reference to particularembodiments thereof. However, it will be appreciated that those skilledin the art, upon consideration of this disclosure, may make modificationand improvements within the spirit and scope of the invention.

1. A ceramic heating element comprising an outer electrical path and arecessed portion to receive an electrical lead.
 2. The ceramic heatingelement of claim 1 wherein the conductive zone has a cross-section for asubstantial length of the heating element that is approximately equal toor greater than the cross-section of the conductive zone at the baseportion of the heating element.
 3. The heating element of claim 1wherein the heating element has a rounded cross-sectional shape for atleast a portion of the heating element length.
 4. The heating element ofclaim 1 wherein the heating element comprises multiple regions ofdiffering electrical resistivity.
 5. The heating element of claim 1wherein the heating element comprises in electrical sequence, a firstconductive zone, a resistive hot zone, and a second conductive zone. 6.The heating element of claim 1 wherein the heating element has asubstantially constant width for at least a substantial portion of theheating element length.
 7. The heating element of claim 1 wherein wallsof the recessed portion comprise portions of a conductive zone andinsulator zone.
 8. The heating element of claim 7 wherein up to 10, 20,30, 40, 50, 60, 70, 80 or 90 percent of the surface area of the recessedportion walls are formed by the conductive zone, with the balance of thesurface area of the recessed portion walls being formed by the insulatorzone.
 9. The heating element of claim 1 wherein the recessed portion isinwardly tapered.
 10. The heating element of claim 1 wherein the heatingelement comprises a first conductive zone and a second conductive zonewith a more resistive ignition zone interposed therebetween, and thefirst conductive zone has a greater length than the second conductivezone.
 11. The heating element of claim 10 wherein the length of secondconductive zone is no more than 90, 80, 70, 60, 50, 40, 30, 20 or 10percent the length of the first conductive zone.
 12. The heating elementof claim 10 wherein the heating element electrical path extends insequence from the first conductive zone to the ignition zone and then tothe second conductive zone.
 13. The heating element of claim 1 whereinthe first conductive zone but not the second conductive zone contactsthe recessed portion.
 14. A method of igniting gaseous fuel, comprisingapplying an electric current across a heating element of claim
 1. 15. Amethod of claim 14 wherein the current has a nominal voltage of 6,8,10,12, 24,120, 220, 230 or 240 volts.
 16. A heating apparatuscomprising a heating element of claim
 1. 17. The apparatus of claim 16wherein the apparatus is cooking unit that comprises an igniter element.18. The apparatus of claim 16 wherein the apparatus is a glow plug. 19.The apparatus of claim 16 wherein the apparatus is a vehicular glowplug.